Security forces used tear gas to disperse protesters in Iran as people angered by the economic situation in the Islamic republic kept up their challenge to the authorities and the exiled opposition Thursday urged them to step up their actions.The 12 days of protests have shaken the clerical authorities under Ayatollah Ali Khamenei already battling economic crisis after years of sanctions and recovering after the June war against Israel.For all the latest headlines, follow our Google News channel online or via the app.The movement, which originated with a shutdown on the Tehran bazaar on December 28 after the rial plunged to record lows, has spread nationwide and is now being marked by larger scale demonstrations.Authorities have blamed unrest on “rioters” and the judiciary chief has vowed there would be “no leniency” in bringing them to justice. On Wednesday, an Iranian police officer was stabbed to death near Tehran “during efforts to control unrest,” the Iranian Fars news agency said